Re: Rules of Evidence (S&T January Editorial)


Pierre Vandevenne wrote:
Chris L Peterson <clp@xxxxxxxxxxxxxxxxxx> wrote in
news:ni5km2lg64h2jj2bid82gljglpqlhecrfh@xxxxxxx:

"enhanced". Details of the actual optical performance were, in fact,
made available, and the RCX design appears to have better optical
performance in many respects than an equivalent sized RC. I also read

An interesting side effect of the lawsuit, at least in my mind: I am
beginning to believe that if RCOS goes to that length, the Meade design
really works well.
[...]

That is true, as Roland Christen of Astro-Physics would concur.

Though I don't expect Astro-Physics to become a Meade dealer and sell
LX200R and RCX400 scopes alongside A-P's refractors and mounts (though
A-P will be selling a lot of optical accessories for the LX200R and
RCX400,
see below), Roland hangs-out in the Yahoo "RCX400" group, had written
some
good things about the optical design, and just wrote this there last
Friday:

The RCX is good Message #9232 of 9255 Fri Nov 24, 2006 2:30 pm
"uncarollo2" <chris1011@...>

I have had a chance now to use a 10" F10 version of the famed
Meade "RC" scopes, the 10" LX200R. I was skeptical that it would
perform on a par with any of the more advanced RC types, so I did not
expect really good results from it. In any case, I made a special
adapter to allow me to use the AP 0.75x telecompressor on the back of
the scope, and mounted an SBIG ST10XE and an STL11k camera. The
ST10XE was not a good match for this scope. The star sizes were
consistently on the order of 10 - 12 pixels FWHM, and the field size
was small, so that the image looked relatively poor when adjusted to
my monitor size. Basically, the image was being way oversampled and
was not taking advantage of the full field size of this scope.

The STL11K camera with its 9 micron pixels was a much better match,
especially when I imaged at 2x2 binning. The field size with the .75x
compressor is close to 1 degree and even at 100% size, the final
image is almost twice as large as my monitor with excellent
sharpness.

I spent one night shooting M33 and posted the result in the files
section.

http://tech.groups.yahoo.com/group/RCX400/files/Astrophotos%20with%20RCX/

under M33 folder. I compared this image to several posted by classic
RC users, and I have to admit, it stacks up quite well, even when
compared to those taken with fairly large RCs. I shot through high
thin clouds, so my signal to noise is not what it could be. Also, my
observatory is located next to the Astro-Physics factory in a light
polluted area with several suburban shopping centers and car
dealerships just down the road. A better test would be to take the
scope out to a dark sky site (if I ever get the time).

I had the scope mounted on the new Astro-Physics Mach 1 mounting, and
I posted another image showing a screen shot of the MaximDL tracking
graph during one of the exposures. Each pixel of the STL11k guide
chip is approximately 0.75 arc sec at the focal length I was using,
so you can see that this little mount really tracks like a champ -
less than 1 arc sec rms.

One other note: Although the telecompressor would theoretically
provide a reduction to F7.5, the actual reduction with the 10" is
closer to F8.5. The reason for that is that the compressor requires
more back focus than what you need at F10. Moving the mirror forward
to get this backfocus results in the scope producing between F11 and
F12, thus there is a kind of tug of war going on between compressor
and moving mirror. In any case, the resultant resolution is
maintained, and actually all the compressed images show slightly
better resolution than the F10 images that I took. (By the way, AP
will be offering an adapter system to allow 2.7" accessories to be
attached on the back of all Meade RCX and LXR scopes).

Vignetting is evident both at F10 and F8.5, but not so bad that it
cannot be fully taken care of with flat fielding. Star images without
the compressor are quite good to the corners. With the compressor the
stars are oval at the extremes. Even so, the stars over the same
field size are pretty much round either way. It's just that the field
is larger with the compressor, and more stars are shown. Cropping the
images slightly results in a very nice field with minimal vignetting
and astigmatism in the corners. Resolution is pretty much identical
either way, but the signal level is significantly higher, which means
shorter exposure for the same density.

Critique of the 10" LX200R: In my professional opinion, there are a
couple of things about this scope that are a negative, and if Meade
changed the design somewhat, they would have a first class system,
kind of unbeatable at the price, even if they raised the price a bit
for the upgrade.

1) the main complaint is that the focuser is so coarse that it is
very difficult to get perfect focus. A 10:1 mechanism would be a nice
addition, or simply using a finer thread on the focuser screw (I may
modify mine with a fine thread).

2) the image shift is quite bad, which prevents going back and forth
during focusing. One must approach from one direction slowly. Adding
a Crayford focuser would solve this, but that eats up more back
focus, and causes the focal ratio to increase yet again.

3) the aluminum tube results in significant focus shift during
temperature changes, and coupled with the poor focuser system it is
hard to take long exposures. The obvious solution is carbon fiber.

4) although the secondary obstruction is quite large, especially on
the main mirror, the rear opening of the scope is quite small. This
prevents placing telecompressor optics closer up the light path. As a
result, there is a limit to the amount of compression that can be
achieved with a high quality telecompressor, especially one that
covers a large field of the STL cameras. Even at F10, the opening is
too small to fully cover the field. I don't know what the situation
is for the 12" and larger scopes, but the 10" has a rear aperture of
only 50mm, whereas the mirror central obstruction is over 90mm in
diameter.

If anyone is interested, I can post a picture of my 10"LX200R - Mach1
setup. Again, I think for the money this is quite a respectable scope
with some real possibilities for high quality astro images.

Rolando

.

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